a) Field of the Invention
The invention relates to a semiconductor device and a method for manufacturing a semiconductor device, particularly a display included a built-in driver circuit which is integrated a thin film transistor (TFT) as a switching element for a display area and also configuring a driver circuit on the panel end, and used for an active matrix display device such as a liquid crystal display (LCD) and an electroluminescent (EL) display.
b) Description of the Related Art
Due to their advantages in terms of compactness, thinness and reduced power consumption, LCDs have come into widespread practical use in the fields of OA equipment and AV equipment in recent years. In particular, an active matrix type LCD with a TFT arranged on each pixel as a switching element for controlling rewriting timing of pixel data can display moving pictures on a large display with high resolution and is used for various types of TVs and as monitors for personal computers and the like.
An EL display device having organic EL as an optical member was also developed to remedy viewing angle-dependent problems of LCDs. TFTs are also used as switching elements to drive each EL element.
A TFT is a field effect transistor (FET), which is obtained by forming a metal layer and a semiconductor layer into a predetermined shape on an insulating substrate. In the active matrix type LCD, the TFT is connected to each capacitor, which is formed between a pair of substrates with liquid crystal sandwiched therebetween to drive the liquid crystal.
Specifically, instead of amorphous silicon (a-Si) which has been used a lot as the semiconductor layer, LCD using polycrystalline silicon (p-Si) was developed. Annealing with laser light is also used for forming or growing p-Si grains. Generally, p-Si has higher mobility than a-Si, a TFT is downsized, and a high aperture ratio and high resolution can be achieved. Since a gate self-align structure can be adopted, a TFT can be made fine, and parasitic capacitance can be decreased. Thus, the TFT can be made fast. An electrical complementary connection structure of n-ch TFT and P-ch TFT, namely CMOS (complementary metal oxide semiconductor), can also be formed, and a high-speed driving circuit can be configured. Therefore, integral formation of the driving circuit in the periphery of the pixel area on the same substrate allows reduction in the manufacturing cost and reduction in size of the LCD module.
Methods for forming a p-Si film on the insulating substrate include annealing for crystallization of a-Si prepared at a low temperature and a solid phase crystallization in a high temperature state, both requiring processing at a high temperature of 600xc2x0 C. or more. Therefore, an inexpensive non-alkali glass substrate cannot be used as the insulating substrate because of its inferior heat resistance, and an expensive quartz glass substrate is required, resulting in high costs. Meanwhile, there is developed a method enabling the use of a non-alkali glass substrate as the insulating substrate. This method employs laser annealing to polycrystallize silicon with a substrate at a relatively low temperature of 600xc2x0 C. or below. Such a process having a processing temperature of 600xc2x0 C. or below throughout the whole process of the TFT substrate production is called a low-temperature process, which is essential for mass-production of low-cost LCDS.
FIG. 1 is a plan view showing relationships between a subject substrate 1 to be processed and irradiating and scanning directions of the excimer laser in the excimer laser annealing (ELA) effected by irradiating laser light. The subject substrate 1 is an ordinary non-alkali glass substrate, which has a-Si formed on its surface. The substrate 1 is a mother glass substrate having six active matrix substrates 5 for constituting an LCD. The individual active matrix substrate 5 comprises a display area 2 having pixels arranged in a matrix at the center, and a gate driver 43 and a drain driver 44 which are formed around the display area 2. In the display area 2, a pixel electrode which is one of the electrodes of a pixel capacitor for driving the liquid crystal is to be arranged in a matrix, and p-Si TFTs which are prepared by polycrystallizing by ELA are connected to them. The gate driver 43 is mainly formed of a shift resister, while the drain driver 44 is mainly formed of a shift resister and a sampling circuit. These drivers 43, 44 are formed of TFT arrays such as CMOS using the p-Si film prepared by polycrystallization by ELA.
A pulse laser is used for ELA, and each pulse laser beam being irradiated has its edge indicated as C having, e.g., a line width of 0.5 to 1.0 mm and a line length of 80 to 150 mm, in FIG. 1. The line beam is moved on the subject substrate 1 while overlapping as predetermined, so that the laser light is fully irradiated to process a large area of the substrate 1, thereby polycrystallizing a-Si.
FIG. 2 shows TFTs formed on the subject substrate 1, and particularly a plan configuration of an inverter portion used at respective parts in the drivers 43, 44. FIG. 3 is a sectional view taken along line Bxe2x80x94B of FIG. 2. A gate electrode 51 connected to an input of the inverter is formed on a transparent substrate 50 of a non-alkali glass substrate or the like, and a gate insulating film 52 is formed to cover the gate electrode 51.
A p-Si film 53, which is formed by ELA, is formed on the gate insulating film 52 like islands to lie across over the gate electrode 51 in N-ch and P-ch areas. The part of the p-Si film 53 just above the gate electrode 51 is a non-doped channel area CH. On the N-ch side, an LD (lightly doped) area LD doped with a low concentration of N-type impurities is formed on both sides of the channel area CH, and a source area NS and a drain area ND, which are doped with a high concentration of N-type impurities, are formed next to the LD areas LD. On the P-ch side, the non-doped channel area CH has on both its sides a source area PS and a drain area PD which are doped with a high concentration of P-type impurities.
An implantation stopper 54 used to form the source and drain areas PS, PD remains on the channel area CH. An interlayer insulating film 55 is formed to cover the p-Si film 53 and the implantation stopper 54. A source electrode 56 and a drain electrode 57 are formed on the interlayer insulating film 55 and connected to the source areas NS, PS and the drain areas ND, PD of the p-Si film 53 through contact holes CT formed in the interlayer insulating film 55. The drain electrode 57 is connected to an output of the inverter, the source electrode 56 on the N-ch side to a low voltage source, and the source electrode 56 on the P-ch side to a high voltage source.
An insulating film 58 having a planarization is formed to fully cover the electrodes. A TFT used as the switching element on the display area 2 is generally an N-ch type and has the same structure as the left sides of FIG. 2 and FIG. 3. A pixel electrode (not shown) for driving the liquid crystal is formed on the planarizating insulating film 58 and connected to the source electrode 56 through the contact holes formed in the planarizating insulating film 58.
FIG. 2 shows particularly the inverter portion of the drivers 43, 44. Such an element related to the logical operation is determined at the time of designing to have a W/L value so to decide performance characteristics. Accordingly, the TFT of N-ch and P-ch shown in FIG. 2 has the island layer of the p-Si film 53 and the gate electrode 51 which are formed to have a width and the like so to fulfill a designed channel width W and a designed channel length L. A single channel area CH having such a value is formed for the individual element.
The p-Si film formed by the excimer laser annealing (ELA) has a disadvantage that a grain size does not become large enough, and a linear area poor in crystallinity is produced in sides of a linear pulse laser beam, particularly in its longitudinal direction, causing stripes as indicated by R in FIG. 1 and FIG. 2.
Such a defective crystallization area R of the p-Si film has poor crystallinity, and TFT formed in the area containing such an area is generally poor in characteristics.
A TFT to be formed on the subject substrate 1, if it was prepared including such a defective crystallization area R, has generally deteriorated element characteristics.
Occurrence of such an area with locally poor crystallinity is assumed to be a result of the following. Where a-Si is crystallized by ELA to prepare p-Si, the laser energy and the grain size are related to each other as shown in FIG. 4. It is apparent from FIG. 4 that the grain size increases up to a given energy value with the increase of energy, but when the energy value exceeds energy Eo for providing the largest grain size, the grain size suddenly becomes small. Therefore, in order to obtain a predetermined grain size GM or more, the laser energy irradiated must be in a range between an upper limit Eu and a lower limit Ed.
However, as shown in FIG. 5, the irradiated line beam has distribution of irradiated light intensity with respect to the position, which is not completely flat in a section A of the line width (in a breadth direction) of the beam line. The line width A specified by an optical mechanism of a laser beam irradiating apparatus substantially has a sharp edge and a distribution shape with a flat energy Eo. However, as indicated by X or Y in FIG. 5, there are portions where the intensity increases and decreases sharply and exceeds an allowable range Ed-Eu of energy to obtain an optimum grain size.
The occurrence of the excessively high portion X and the excessively low portion Y in the irradiated energy is assumed to mainly result from particles or the like adhering to any lenses of the optical system of the laser irradiation apparatus. They cause shading, diffraction, interference or the like, leading to uneven intensity, which is extended in the direction of the line length after the optical system converges the laser in a direction of the line width. Thus, if the particles causing the inconsistencies in the light are present in a clean room even in a very small amount, the optical characteristics are affected, and the flat distribution of light intensity is degraded. For the time being, it is difficult to make the characteristic shown in FIG. 5 fully flat by thoroughly preventing the adhesion of such particles. Therefore, an area having defective crystallinity can be prevented from being formed in a direction along the linear pulse laser, particularly along the long sides.
Further, the irradiated energy is variable even among the shots of pulse laser beams, and the defective crystallization area R is produced or not on the subject substrate 1. Further, when the irradiated energy of a given shot of the line beam from the pulse laser is out of the optimum range Ed-Eu, no shot comes after at the last end portion in the scanning direction of the line beam, and crystallinity is not restored. Consequently, a linear detective crystallization area R is formed.
A TFT having the structure as shown in FIG. 2 is formed on the subject substrate 1 of FIG. 1. In this TFT, for example, when the LCD is constructed, the channel area CH is formed at the intersection of the gate electrode 51 formed in a horizontal scanning direction H or a vertical scanning direction V (horizontal scanning direction H in FIG. 2) and the p-Si film 53 formed across the gate electrode 51. In this channel area CH, an electrical charge being controlled for conduction/non-conduction is moved through a channel connecting the source areas NS, PS and the drain areas ND, PD. Also, as shown in FIG. 2, the channel area CH has the channel length L in the vertical direction of the drawing and the vertical scanning direction V on the LCD. The channel width W is in the horizontal direction in the drawing and in the horizontal scanning direction H on the LCD. In the configuration as described above, where the defective crystallization area R occurs in the direction as shown in FIG. 2, it may happen that a width T of the defective crystallization area R is larger than a channel width W of the channel area CH, and the defective crystallization area R may occupy most of the channel area CH. The TFT""s performance characteristics are also degraded compared with another TFT. Since these TFTs are used by the drivers 43, 44 which drive the pixels of the LCD, the degradation of the performance characteristics of the TFTs leads to degradation of the display quality such as a shift of drive timing or variations in display characteristics of given lines or columns of the display area 2.
The invention has been achieved to remedy the above-described disadvantages and has the following characteristics.
A display device comprises, a plurality of pixel electrodes formed on a substrate; a plurality of first thin film transistors, which are connected to corresponding pixel electrodes among the plurality of pixel electrodes and respectively supply the connected pixel electrodes with a display signal; and a plurality of second thin film transistors, which constitute a driving circuit for driving the plurality of first thin film transistors; wherein some or all of the plurality of second thin film transistors have a plurality of channel areas formed in a semiconductor layer subjected to laser annealing respectively, and the plurality of channel areas are electrically connected in parallel to each other and arranged separately.
When the laser annealing is performed to improve the quality of the semiconductor layer, such as obtaining a polycrystallized semiconductor layer by polycrystallizing, for example, an amorphous semiconductor layer, a defectively annealed area extending in a certain direction is formed in the semiconductor layer and overlaid on some of the plurality of channel areas constituting one semiconductor element, and the pertinent portions have defective performance characteristics. However, by configuring as described above, the other channel area of the same semiconductor element is highly likely to be excluded from the defectively processed area. Therefore, the characteristics of the semiconductor element as a whole are not degraded, and electrical operation can be carried out normally. Accordingly, where the present invention is applied to, for example, a liquid crystal display, a high-performance p-Si TFT LCD with drivers built in can be obtained.
Thus, by configuring as described above, even if a defectively processed area is overlaid on any channel area, the possibility of overlaying the defectively processed area on the other channel area becomes very low.
Furthermore, according to another aspect of the invention, the plurality of channel areas are separated in a direction of the cannel width.
Thus, the channel area which becomes a defectively processed area is reduced, and the element having a larger channel width can be obtained.
Another aspect of the invention relates to a display, which comprises, a plurality of pixel electrodes formed on the substrate; a plurality of first thin film transistors, which are connected to corresponding pixel electrodes among the plurality of pixel electrodes and respectively supply the connected pixel electrodes with a display signal; and a plurality of second thin film transistors, which constitute a driving circuit for driving the plurality of first thin film transistors; wherein some or all of the plurality of second thin film transistors have a plurality of channel areas formed in a semiconductor layer subjected to laser annealing respectively, and the plurality of channel areas are electrically connected in parallel to each other and arranged in different directions.
The plurality of channel areas can be arranged so to be made orthogonal to each other in a direction of the channel width. Also, the plurality of channel areas can be formed in the same island semiconductor layer or arranged separately to each other.
Still another aspect of the invention relates to a semiconductor device having a plurality of semiconductor elements on a substrate, wherein some or all of the semiconductor elements have a plurality of channel areas which are formed in a semiconductor layer subjected to laser annealing respectively, and the plurality of channel areas are electrically connected in parallel to each other and arranged separately and/or arranged in different directions to each other.
In addition to the display described above, a semiconductor device having such a plurality of semiconductor elements can also prevent the characteristics of the semiconductor elements from being degraded due to a defectively processed area caused in the same direction on the semiconductor layer which is laser-annealed as described above.